Organizational or business users of telecommunications services have communications needs that may not be satisfied by the ordinary subscriber network. Typically such service is provided by a private branch exchange (PBX or PABX) system or a centrex System. Centrex service refers to a type of central office service that provides the equivalent of PBX or PABX service furnished by telephone companies through equipment located in the central office (CO). Centrex service provides direct inward dialing (DID) and direct outward dialing (DOD) and is attractive in that it requires a minimum amount of telephone equipment located at the customer premise.
In centrex telephone exchanges, the management of telephone calls to and from the telephone stations served by exchange is computer processor controlled and driven. In one form the exchange provides a multistation pick up capability. In this type of exchange, the memory of the centrex is provided with a map which defines secondary telephone stations of the exchange which can pick up telephone calls originating at each station of the exchange. Each station thus defines with these secondary stations a so called multi-station pick up group.
Hereinafter, when referring to the stations of a multi-station pick up group, the term primary station is used to refer to the station whose call can be picked up and the term secondary station is used to refer to each station which can pick up for the primary station. Any telephone station in the exchange might be a primary station of one pick up group and a secondary station of one or more other pick up groups.
When the centrex exchange originates a call at a primary station of a pick up group, i.e., rings that station, and the primary station does not pick up, any of the secondary stations in the pick up group can then pick up the call. Pick up is initiated by a secondary station transmitting a pick up signal to the centrex exchange. This pick up signal is typically generated by preselected keying at the secondary station.
The centrex exchange receives the pick up signal and processes the signal via its pick up group map. In particular, the centrex determines from this map whether the secondary station identified by the pick up signal is in the pick up group of the called primary station. If the secondary station is in this pick up group, the centrex then transfers the call from the called station to the secondary station and the call is then picked up.
In order for the secondary telephone stations in each pick up group to be aware of the call status at the primary station of the group, each secondary station may be provided with a lamp or other indicator which identifies the primary station and whose state indicates the status of such station. Control of these lamps is through the centrex exchange. Typically the centrex exchange monitors the status of the primary station in each pick up group and transmits to the secondary stations in the group lamp control signals which control the states of the lamps of the secondary stations in accordance with the monitored status (e.g., primary station busy----secondary station lamp is controlled to be full on; primary station ringing----secondary station lamp is controlled to be flashing at specified rate; etc.).
Despite these features it has been found that in actual practice centrex users typically install some form of key or feature telephone behind the centrex in order to obtain the desired service. The reasons for this are complex but basically fall into one or more of the following categories:
1. Need for additional line appearances on one or more telephones. PA1 2. Need for signaling or intercommunication between one or more telephones in a group (intercom). PA1 3. Need for easy access to features of the centrex system (button access). PA1 1. Their features clash with those of the centrex system, i.e., transfer of calls. PA1 2. With electronic equipment a lack of flexibility for provision of line appearances. PA1 3. Equipment cost along with the maintenance service associated with such equipment.
In order to satisfy these requirements vendors generally install electronic key systems and in some instances install electromechanical equipment of old if not obsolete design. In either event such systems were not specifically designed to work with centrex and generally encounter one or more of the following problems:
As one alternative to the centrex controlled system it has been proposed to use modular station controllers between the telephone stations and centrex exchange (David Systems Corporation). These station controllers are each assigned a number of stations and each permits digital communication with its assigned stations. Multi-station pick up is accomplished by providing both lamp and switching control for the pick up groups at the controllers. This requires comprehensive switching equipment at each controller. Also in this type of system additional per station relay equipment is utilized to enable telephone stations not assigned to a controller to be in a pick up group controlled by that controller. While this approach reduces the processing constraints on the centrex exchange and allows the use of digital telephone stations, the switching equipment needed in each controller is complex and costly.
In another alternative described in U.S. Pat. No. 4,885,769 issued 12/5/89 to John D. Beierle, modular station controllers are associated with the centrex central office and connected between the telephone stations and the centrex exchange. According to this arrangement each controller is assigned a group of telephone stations and configured and programmed to permit digital communication with its assigned stations. Lamp control for the stations within a group is accomplished by the controller for the group. Lamp control for the total stations collectively is accomplished through a local area network (LAN) linking the controllers. The switching occurs in the centrex for all communication, i.e., between stations in a group, call transfer within and across groups, all call pick up and intercom. The station controllers provide digital communication between the assigned telephone stations and the controller and provide analog communication between the controller and the centrex exchange. In the described system this involves analog-to-digital (A/D) and digital-to-analog (D/A) conversion in each station line served by each controller.